1. Field of the Invention
The present invention relates to a technical field of a method and an apparatus for image recording by means of a top shooter type thermal ink jet system, and more particularly to an ink jet recording method and an ink jet recording apparatus that are capable of changing ink droplet ejection amounts in thermal ink jet system of atmosphere communication type, thereby achieving a uniform ink droplet ejection amount among nozzles and finer gradation.
2. Description of the Related Art
So-called thermal ink jet recording system in which a part of ink is rapidly vaporized through the application of heat by a heater and the ink (ink droplet) is ejected from a nozzle by the expansive force thereof or the like, is applied to various kinds of printers (see JP 48-9622 A, JP 54-51837 A).
In such a thermal ink jet system, the gradation of an image is expressed by adjusting the number of ink droplets to be ejected for one pixel, that is, by performing area modulation. With this method, however, the expressible graduation is limited in accordance with the resolution (spatial resolution) of an image and the like. In addition, the limit of the expressible gradation is low compared with image recording that uses a photosensitive material or the like.
In view of this problem, there has been studied a technique of enhancing the expression ability of gradation by changing an ejection amount of ink in the thermal ink jet system.
As a method of changing an ink ejection amount, there has been mainly proposed a method with which a plurality of heaters and nozzles are provided so as to correspond to one dot (the minimum expression unit) of ink and the numbers of heaters and nozzles to be driven are selected in accordance with an intended ink ejection amount.
With this method, however, it is required to use many heaters and nozzles whose numbers exceed the resolution of a recording head, so that in the case of a high-resolution recording head, it is required to perform extremely elaborate and high-density manufacturing processes, which leads to an increase in cost and a reduction in yield.
On the other hand, there have been also proposed various kinds of methods with which an ink ejection amount for one ink droplet is changed using one heater.
In JP 05-77422 A, for instance, there is disclosed an ink jet recording system in which the area of a nozzle opening is set larger than the area of a heater, thereby allowing an air bubble to grow until the air bubble exceeds the top end of a nozzle. The size of the air bubble is adjusted by adjusting energy supplied to the heater. By doing so, there is adjusted an ink ejection amount. With this method, however, the nozzle area is large compared with the heater, so that there occurs a loss in a portion of the growing energy of the air bubble, which is used for operations other than the formation of a droplet and the ejection of the droplet. Also, there is a fear that the direction in which the ink flies will become unstable.
Also, in JP 2000-141663 A, there is disclosed a technique with which there is formed for a heater a protective layer (passivation film) that includes areas having different thicknesses. With this construction, the amount of an ink droplet to be ejected is adjusted by applying a voltage to the heater in a multilevel manner and controlling a heater area that reaches the nucleate boiling temperature using the applied voltage.
With this method, however, there is formed a protective layer that includes areas having different thicknesses, which leads to an increase in the number of manufacturing processes and an increase in cost.
Further, JP 07-232441 A discloses a side shooter type ink jet recording apparatus in which a time interval from the preceding drive pulse is measured and intervals at which the ink ejection drive means is driven are controlled based on the measured time interval and the gradation information contained in the information to be printed.
It should be noted here that this recording system was made based on the finding that the size of an ink droplet that can be ejected at the following drive pulse can be changed for example to 40 to 100% in size of a common ink droplet by changing the time interval from the preceding drive pulse.
However, a first problem of the ink jet system of not atmosphere communication type but side shooter type disclosed in JP 07-232441 A is that the size of the ink droplet cannot be determined in a constant manner and is prone to be affected by the head structure corresponding to each ink jet nozzle in an ink jet head. Therefore, the prior art ink jet system suffers from the problem that its commercialization is difficult due to large variations in the individual head structures and also in the ink jet as a whole.
A second problem is that changes in the time interval between drive pulses may bring about variations in the ejection timing at which an ink droplet is ejected from each ink jet nozzle, which results in variations in the recording intervals on a recording medium leading to a marked reduction of the printing quality.
In order to solve this second problem, in the ink jet recording apparatus and method disclosed in JP 07-232441 A, a drive pulse having a minimal pulse width sufficient to always eject an ink droplet from an ink jet nozzle is used as an initial pulse; the initial pulse is regarded as the preceding drive pulse and thereafter main drive pulses are given at arbitrary time intervals to thereby controlling the variations in the recording intervals on the recording medium and preventing the printing quality from being markedly reduced.
Nevertheless, this is not effective in solving the first problem described above. The second problem is solved to some extent but the ink ejection drive means must be divided into two means including a first ink ejection drive means for driving at the initial pulse and a second ink ejection drive means for driving at the main drive pulse due to the difficulty in ejecting an ink droplet of minimal amount at the initial pulse in a positive and stabilized manner. In other words, there remains a problem that a special ink ejection drive means for the initial pulse is necessary in addition to the commonly used ink ejection drive means.
An object of the present invention is to solve the problems of the conventional techniques described above by providing an ink jet recording method that is capable of adjusting ink droplet ejection amounts using one ordinary heater (there are included construction elements such as a protective film), thereby correcting variations in ink ejection amounts among heaters and expressing with finer gradation in the so-called thermal ink jet system of atmosphere communication and top shooter type where an air bubble for ejecting ink is made to grow until the air bubble contacts the atmosphere.
Another object of the present invention is to provide an ink jet recording apparatus in which the ink jet recording method described above is implemented.